Query Runs OK In QueryAnalyzer, But Fails In Batch

A colleague of mine has a query which fails to run under SQLAgent
batch with the following error:

The conversion of a char data type to a datetime data type resulted in
an out-of-range datetime value. [SQLSTATE 22007] (Error 242) The
statement has been terminated. [SQLSTATE 01000] (Error 3621). The
step failed.

He can run the same query sucessfully via query analyzer (i.e. no
errors, and it does what he wants)

If I try to run the same query through Query Analyzer on my
workstation, I get a different error altogether:

Server: Msg 242, Level 16, State 3, Procedure DateForGrouping, Line 11
The conversion of a char data type to a datetime data type resulted in
an out-of-range datetime value.

Any idea what might be causing these differences in behaviour
depending on how and/or where the query is run from?

The (working) statement in question is:

================================================== ======================
insert into Summary_ReferrerSales
select DateCreated,
ReferrerID,
ReferrerIDCount,
PUIDCount,
ReferrerDescription,
0 as TotalOrderValue,
0 as TotalOrderLines
from vw_ReferrerPopularity

Warning: Null value is eliminated by an aggregate or other SET
operation.

(11996 row(s) affected)
================================================== ====================
And the table / view / function definitions (I take no responsibility
for the view definition!) are:

CREATE TABLE [Summary_ReferrerSales] (
[DateCreated] [datetime] NULL ,
[ReferrerID] [char] (2) NULL ,
[ReferrerIDCount] [int] NULL ,
[PUIDCount] [int] NULL ,
[ReferrerDescription] [nchar] (100) NULL ,
[TotalOrderValue] [numeric](18, 0) NULL ,
[TotalOrderLines] [numeric](18, 0) NULL
) ON [PRIMARY]


CREATE VIEW dbo.vw_ReferrerPopularity
AS
SELECT TOP 100 PERCENT COUNT(dbo.LogReferrerID.Referrer) AS
ReferrerIDCount,
COUNT(DISTINCT dbo.LogReferrerID.PUID) AS PUIDCount,
dbo.DateForGrouping(dbo.LogReferrerID.DateUsed)
AS DateCreated,
dbo.LogReferrerID.Referrer AS ReferrerID,
dbo.LookupReferrerID.ReferrerDescription
FROMdbo.LogReferrerID INNER JOIN dbo.LookupReferrerID
ON dbo.LogReferrerID.Referrer = dbo.LookupReferrerID.ReferrerID
WHERE (dbo.LogReferrerID.DateUsed > CONVERT(DATETIME, '2003-09-01
00:00:00', 102))
GROUP BY dbo.LogReferrerID.Referrer,
dbo.DateForGrouping(dbo.LogReferrerID.DateUsed),
dbo.LookupReferrerID.ReferrerDescription
HAVING (dbo.LogReferrerID.Referrer <> 'WS')
AND (COUNT(dbo.LogReferrerID.Referrer) IS NOT NULL)
AND (dbo.LookupReferrerID.ReferrerDescription IS NOT NULL)
AND (dbo.DateForGrouping(dbo.LogReferrerID.DateUsed) >
CONVERT(DATETIME, '2003-09-01 00:00:00', 102))
AND (COUNT(DISTINCT dbo.LogReferrerID.PUID) IS NOT NULL)
ORDER BY dbo.DateForGrouping(dbo.LogReferrerID.DateUsed) DESC


CREATE TABLE [LogReferrerID] (
[LogReferrerID] [int] NULL ,
[Referrer] [varchar] (2) NULL ,
[DateUsed] [smalldatetime] NULL ,
[HTTPReferrer] [varchar] (1000) NULL ,
[TargetURL] [varchar] (1000) NULL ,
[QueryString] [varchar] (1000) NULL ,
[PUID] [varchar] (50) NULL
) ON [PRIMARY]


CREATE TABLE [LookupReferrerID] (
[ReferrerID] [char] (2) NOT NULL ,
[ReferrerDescription] [varchar] (100) NOT NULL ,
CONSTRAINT [PK_Lookup_ReferrerID] PRIMARY KEY CLUSTERED
(
[ReferrerID]
) ON [PRIMARY]
) ON [PRIMARY]


CREATE FUNCTION dbo.DateForGrouping (@RowDate datetime)
RETURNS datetime AS
BEGIN

declare @datestring char(10)

set @datestring=cast(datepart(dd,@RowDate) as char(2)) + '/' +
cast(datepart(mm,@RowDate) as char(2)) + '/' +
cast(datepart(yyyy,@RowDate) as char(4))

return cast(@datestring as datetime)
END